Since polybutene-1 resins are excellent in creep resistance, stress cracking resistance, impact resistance, heat resistance, chemical resistance and the like and are flexible, use thereof has been actively developed, along with use of polyethylene resins and polypropylene resins.
The polybutene-1 resin is a useful resin that finds wide range of uses because of the above excellent properties. More specifically, said resin can be molded into various pipes, filaments and films by extrusion molding, into cups and containers by injection molding and into various bottles by blow molding.
However, the polybutene-1 resin forms several crystal modifications or crystal forms and, when the melt of said resin is cooled to the melting point or a lower temperature, unstable form II or the tetragonal crystal modification (hereinafter briefly referred to as "F-II") forms first and then slowly undergoes solid phase transformation into Form-I or the hexagonal crystal modification (hereinafter briefly referred to as "F-I") with a lapse of time. The formation of F-II involves a high degree of supercooling and the rate of crystallization is slow, resulting in retardation of molding cycle. Furthermore, F-II to F-I transformation proceeds at a low rate, and thus it takes a prolonged period of time, usually about a week to 10 days, to complete the transformation.
The crystal modification or crystal form having commercially useful contemplated excellent properties is F-I. On the other hand, there exists a low-melting point crystal modification (Form-III, hereinafter referred to as "F-III") which is presumably rhombic and can be obtained by precipitation from a solution thereof or by casting of a solution thereof.
The crystallization behavior peculiar to the polybutene-1 resin causes various problems in the molding process. For example, when conducting extrusion molding, the molded pipes, sheets and the like must be allowed to stand at room temperature for a prolonged period, partly because the F-II formed immediately after the molding process are flexible and the formed articles, when subjected to an external force during transportation and the like, are likely to be deformed, and partly because it is difficult to obtain molded articles of a prescribed dimension due to the difference in the shrinkage degree among said molded articles if there is a large temperature variation during the transformation. Thus, it is preferable that the molded articles, just after the completion of the molding operation, are allowed to stand in a warehouse which is substantially free of a temperature variation. The period of standing depends on the molding conditions, shape and volume of the molded product and the like, but usually is several days. Thus, the extrusion molding of the polybutene-1 resin is very inefficient.
Further, in the case of injection molding, the molecular orientation is likely to occur which results in anisotropy of the strength, and the molded article may shrink by 1 to 2% after standing at room temperature and thereby become deformed or distorted.
Consequently, in the process of molding the polybutene-1 resin, it is demanded to accelerate the F-II to F-I transformation which is initiated immediately after molding.
Known methods for accelerating the crystallization from a melt of the polybutene-1 resin during the molding process comprise adding stearic acid amide, 1-naphthalene acetamide, benzamide, N,N'-ethylene-bis-stearamide and the like (U.S. Pat. No. 4,320,209, U.S. Pat. No. 4,322,503 and U.S. Pat. No. 4,645,792).
However, the compounds disclosed in these U.S. patents do not produce satisfactory crystallization acceleration effect, and all of these patents disclose only acceleration of the crystallization rate from a melt of the polybutene-1 resin and do not teach the acceleration of the F-II to F-I transformation.
As a method for accelerating the F-II to F-I transformation, it is known to add polypropylene, polyethylene or a sorbitol derivative. However, among these additives, polypropylene and polyethylene must be added in a large amount and are likely to result in poor properties, and the sorbitol derivative tends to exhibit insufficient effects.
Known methods further include a method comprising immersing the molded product in an organic solvent immediately after molding, a method comprising applying an external stress such as compression or stretch to the molded article immediately after molding, and other methods. However, the immersion in an organic solvent is not practically advantageous since it requires special equipment and the solvent after the treatment must be removed by drying and collected. The application of an external stress can not be employed for an article having an intricate shape.
Increase in the formation of F-I fraction immediately after molding is effective for shortening the period to complete the crystal phase transformation, inhibiting the shrinkage of the molded article, preventing the deformation or distortion of the molded article, and other purposes. However, substantially no method therefor has been proposed.
Additionally, an attempt has been made to impart low-temperature heat sealing properties to the polybutene-1 resin, for example, by blending said polybutene-1 resin with a ethylene-propylene random copolymer. However, there arises a problem that the low-temperature heat sealing properties are deteriorated with a lapse of time under the influence of the crystal transformation of the polybutene-1 resin which occurs after the formation of the films.
Therefore, there is a demand for a polybutene-1 resin composition which can give a molded article that shows excellent low-temperature heat sealing properties which would not be deteriorated with a lapse of time.